I had a trooper in ID stop me and read me the riot act over a number of things (over width, which I was not; over weight which I was not) including having an 80 gallon semi fuel tank permanently mounted in the bed of my F250 for gasoline.  He said it had to be "plumbed" into the system, which it was,through a 3 way valve.

I don't know by what authority he made that claim, since it was less than 110 gals.  He was so mad after measuring my load and my pointing out my tank plumbing that he made me run back a round trip of 50 to 60 miles to run me across the scales at the port.  There was more than one arrogant ID trooper on duty that evening.   I walked out unhappy because I was waylaid, but glad that I was not ticketed.  YES, I should have had a logbook.  I did have my medical card.  The sign said to stop at the port IF I was over 29,000 lbs; I was less than 26,000 but apparently should have stopped.

So, yes, you may be "legal" if you jump through all of the hoops, including maximum gallons, but SAFETY is a bigger issue.  It may be better, and a lot less hassle, to support the local economies as you travel.  Put in your bbq/smoker instead of the fuel tank, or leave a little room just in case the MRS wants to do some shopping.

Have a safe trip.